Reference: Einstein’s 1920 Book
Section IX (Part 1)
The Relativity of Simultaneity
Please see Section IX at the link above.
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Summary
Events that are simultaneous with reference to a coordinate system at rest are not simultaneous with respect to a coordinate system in motion. Every coordinate system has its own particular time. Unless we are told the coordinate system to which the statement of time refers, there is no meaning in a statement of the time of an event.
Time has been considered to be independent of the state of motion of the coordinate system. But this is not so because of the velocity of light is not infinite. If we take the finiteness of the velocity of light into account, then the conflict between the law of the propagation of light in vacuo and the principle of relativity disappears.
The duration of a particular occurrence with respect to a coordinate system in motion must not be considered equal to the duration of the same occurrence as judged from the coordinate system at rest.
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Comments
The sense of time changes with the motion of a coordinate system.
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